Field of the Invention
The present invention relates to the field of magnetic resonance imaging, in particular to a body coil for a magnetic resonance imaging apparatus, and a magnetic resonance imaging apparatus having such a body coil.
Description of the Prior Art
Magnetic resonance imaging (MRI) is a modality in which the phenomenon of magnetic resonance is used to perform imaging. MRI systems generally have a superconducting magnet forming a patient receptacle, gradient coils arranged inside the superconducting magnet encircling the receptacle, a body coil located inside the gradient coils and also encircling the receptacle, an examination table on which a patient is placed, and a local coil for covering a particular part of the patient. The body coil may serve as both a transmitting coil and a receiving coil; the local coil chiefly serves as a receiving coil. The superconducting magnet provides a uniform basic magnetic field B0 in the axial direction of the magnet. The basic magnetic field B0 is static and has a high field strength. When the body coil is used as a transmitting coil, it transmits radio frequency (RF) signals, and produces a magnetic field B1 that is perpendicular to magnetic field B0. The magnetic field B1 has a lower field strength than B0, and is generated only during operation. The B1 field is an oscillating field, rotating with a frequency γ*B0. The frequency is about 64 MHz when B0 is 1.5 T. Once excited, hydrogen atoms in water molecules in the body are deflected, then return to the “base state” while emitting an RF signal called a magnetic resonance signal. A receiving coil receives this signal by induction. In order to distinguish between the positions of different hydrogen atoms in space, the gradient coils generate gradient magnetic fields of different strengths in three spatial directions, such that the strength of the magnetic field at any point in space is unique. By subjecting a signal received by the receiving coil to calculations based on the gradient magnetic fields, a signal for a corresponding position is obtained.
A birdcage coil is a commonly used type of body coil. A birdcage coil is formed by end rings at opposite ends and multiple rods therebetween, and may be one of three types, namely high-pass, low-pass and band-pass. Depending on the type of body coil, a frequency tuning capacitor will be incorporated on the two end rings and the antenna rods. For instance, the frequency tuning capacitors of a low-pass birdcage coil are located in the middle of each rod, while the frequency tuning capacitors of a high-pass birdcage coil are located on metal rings at the two ends. In the case of a band-pass birdcage coil, the frequency tuning capacitors may be located on both the rods and the end rings. In general, one or more switch diodes, such as a PIN diode, is/are series-connected on each rod, or on some of the rods, and/or on the end rings of a birdcage coil, to serve as a body coil switch. When the MRI system emits an RF signal, the PIN diode is powered by a forward current, so the PIN diode is conducting, and the body coil is tuned. When the MRI system receives an RF signal, the PIN diode of the body coil is reverse biased, so the PIN diode is non-conducting, and the body coil is detuned.
FIG. 1 is a schematic illustration of a known existing body coil. As FIG. 1 shows, the body coil is a high-pass birdcage quadrature coil. The birdcage coil has 16 rods, with one PIN diode series-connected on each rod. When the MRI system emits an RF signal, the 16 PIN diodes need a total current of 4.8 A as a power supply, with an average of 300 mA allocated to each PIN diode, so that the PIN diodes are conducting, and the body coil is tuned. When the MRI system receives an RF signal, each PIN diode is powered by a reverse voltage of −31 V, so that the PIN diodes are non-conducting, and the body coil is detuned.
Although body coil tuning/detuning schemes commonly used at present are problem-free in terms of function, and widely applied, the high power supply current of the PIN diodes makes the cost of the DC power supply high. Moreover, when the body coil is in a tuned state, despite the fact that sometimes no RF energy is emitted or only a very small amount of RF energy is emitted, it is still necessary to continuously supply a DC current of 4.8, which results in an unnecessary DC loss, and is likely to cause the problem of the PIN diodes generating heat.